Piezoelectric Polarization Effects Research Articles

Observation of hole accumulation at the interface of an undoped InGaN/GaN heterostructure

A pronounced hump structure at about −5 V in the high-frequency capacitance-voltage (C-V) curve of an undoped InGaN/GaN heterostructure is observed and this hump weakens gradually with decreasing measurement frequency, indicating the occurrence of an inversion behavior in the InGaN/GaN heterostructure. The inversion behavior in the C-V curve is attributed to hole accumulation at the heterointerface where a hole well is formed due to the strong piezoelectric polarization effect in the InGaN/GaN heterostructure. The acceptor traps related to Ga vacancies in the InGaN layer are thought to be the source of the minority carriers. The theoretical calculation of band diagram of the InGaN/GaN heterostructure confirms the formation of the hole well at the heterointerface and supports the behavior of hole accumulation under negative bias voltage.

RF performance assessment of AlGaN/GaN MISHFET at high temperatures for improved power and pinch‐off characteristics

AbstractAn investigation of temperature model for an AlGaN/GaN MISHFET is presented and a relative comparison is done with conventional HFET structures. The proposed analytical model demonstrates its inherent ability to operate at higher temperature. The contributions from various temperature dependent material parameters are taken into account alongwith the effect of temperature on the sheet carrier concentration and threshold voltage of the device. The model is further extended to predict the temperature dependence of transconductance, cut off frequency and pinch‐off characteristics. The investigated temperature range is from 25°C–300°C. Non linear Fermi potential (Ef) variation with the sheet carrier concentration (ns), and the highly dominant effects of spontaneous and piezoelectric polarization at the AlGaN/GaN heterointerface are also considered. AlGaN/GaN MISHFETs demonstrate larger drain currents, cut‐off frequency and better pinch‐off characteristics at high temperatures. The model is based on closed form expression and does not involve elaborate computation. The analytical results on the transport characteristics of both the devices are compared with available experimental data and are in good agreement. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1942–1949, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24461

Effect of spontaneous and piezoelectric polarization on optical characteristics of ultraviolet AlGaInN light-emitting diodes

Polarization is a crucial issue for electrical and optical characteristics of the UV AlGaInN light-emitting diodes. The effect of spontaneous and piezoelectric polarization on optical characteristics of the UV AlGaInN light-emitting diodes is investigated numerically. The simulation results indicate that the polarization-related effect for the UV AlGaInN light-emitting diodes is dominated by the piezoelectric polarization. When the UV AlGaInN light-emitting diodes are without piezoelectric polarization, the optical performance is enhanced effectively due to improved overlap of electron and hole wavefunctions, reduced electron leakage current, and increased density of holes in the active region.

Influences of Polarization Effects in the Electrical Properties of Polycrystalline MgZnO/ZnO Heterostructure

AbstractZnO has shown great promise for the application in optoelectronic devices. Since the modulation of conductivity is one of the key issues in device performances, we have applied the Monte Carlo method to analyze the mobility of poly-crystalline MgZnO/ZnO heterostructure thin film layer in this paper. The effects of the grain boundary scattering, ionized impurity scattering, as well as phonon scattering are considered. Our study shows that with a design of modulation doping by including the effects of spontaneous and piezoelectric polarization, the grain boundary potential can be suppressed to improve the mobility of the ZnO layer by order(s) of magnitude. Simulation results are also confirmed by our experimental works that polarization effects play an important role to attract carriers and to increase the mobility.

Electronic and optical properties of InGaN quantum dot based light emitters for solid state lighting

In this paper, we have made a systematic study of the electronic and optical properties of InGaN based quantum dot light emitters. The valence force field model and 6×6k⋅p method have been applied to study the band structures in InGaN or InN quantum dot devices. Piezoelectric and spontaneous polarization effects are included. A comparison with InGaN quantum wells shows that InGaN quantum dots can provide better electron-hole overlap and reduce radiative lifetime. We also find that variation in dot sizes can lead to emission spectrum that can cover the whole visible light range. For high carrier density injection conditions, a self-consistent method for solving quantum dot devices is applied for better estimation of device performance. Consequences of variations in dot sizes, shapes, and composition have been studied in this paper. The results suggest that InGaN quantum dots would have superior performance in white light emitters.

Enhancement of Intersubband Absorption in GaInN/AlInN Quantum Wells

Recently, there has been increasing interest in III-nitride semiconductors for the fabrication of ultrahigh-speed intersubband (ISB) devices. First of all it is due to the large conduction band discontinuity available in these materials. The very deep quantum wells (QW) which can be fabricated, are used as an active region for ISB devices for telecommunication applications (1.55 and 1.3 μm). Moreover the ISB absorption recovery times in nitride semiconductors are extremely short, typically of the order of 150-400 fs due to enhanced electron-phonon interaction in these highly ionic materials [1]. One can expect that ISB devices could provide ultrafast data transfer in a single channel of optical fiber. The predicted speed should be in the range of 0.1-1 Tbit/s bit-rate regime, which is significantly larger than the speed achieved with current bipolar technology. Our work has focused on the growth of GaInN/AlInN multi QWs on GaN substrates. Properly choosing growth parameters we were able to fabricate high quality ISB structures exhibiting intense ISB absorptions at 1.55 μm [2]. Due to a partially strain compensation even quite thick structures were crack free. However, an application of such structures in highspeed ISB devices requires extremely good optical and electrical parameters, which should be appropriate for μ-size devices. In the present work we show a significant enhancement of ISB absorption for 15.5 A thick GaInN QWs due to increase of the AlInN barrier width. The enhancement of ISB absorption was more then 3 times for enlarged barrier width from 3 to 6 nm. Also, we observe that the barrier width does not influence the energy of interband and intersubband transitions within experimental accuracy. The experimental results have been compared with theoretical calculations which were performed within the electron effective mass approximation solving the Schrodinger and Poisson equations self-consistently. Strain effects as well as the effect of spontaneous and piezoelectric polarization have been included in these calculations. A good agreement between experimental data and theoretical calculations has been observed for the investigated samples. By analyzing the electron wave functions, it has been concluded that the electrons significantly penetrate the quantum well barriers and that the QWs are coupled despite the fact that the energy coupling between QW levels is very weak. The observed enhancement of ISB transitions can be associated with the effect of better localization of the second electron level in QW. This finding seems to be very important and useful to optimize ISB devices. In this paper this issue will be discussed in details. This work was partially supported by the Polish Ministry of Science and Higher Education as scientific projects Nos. N507 011 32/0500 and N202 41090 33.

Modeling and analysis of fully strained and partially relaxed lattice mismatched AlGaN/GaN HEMT for high temperature applications

The paper presents an accurate charge control model for a fully strained (FS), and partially relaxed (PR) lattice mismatched AlGaN/GaN HEMT, taking into consideration the effect of spontaneous and piezoelectric polarization. The model also accounts for the mobility degradation with increase in temperature, which is one of the major causes in deterioration of the driving current. By using the variation of band gap with temperature, the temperature dependence on threshold voltage, sheet carrier concentration and drain current is studied. Further, the temperature variation shows the applicability of the device in a variable thermal environment. A close agreement of calculated data with simulated/experimental data proves the validity of the model.

Free‐standing zinc‐blende (cubic) GaN substrates grown by a molecular beam epitaxy process

AbstractIn this paper bulk, free‐standing, zinc‐blende (cubic) GaN wafers grown by plasma‐assisted molecular beam epitaxy (PA‐MBE) are described. GaN layers of up to 60 microns in thickness have been grown. Characterisation measurements confirm the cubic nature of the GaN crystals and show that the fraction of hexagonal material present is not more than ∼10%. Cubic (001)GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0001) c‐axis wurtzite GaN. Free standing GaN wafers grown by PA‐MBE make ideal lattice‐matched substrates for the growth of cubic GaN‐based structures for optoelectronic devices, and high power and high frequency electronic applications. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spontaneous and Piezoelectric Polarization Effects on the Frequency Response of Wurtzite Aluminium Gallium Nitride / Silicon Carbide Heterojunction Bipolar Transistors

ABSTRACTWe present theoretical calculations for the effect of spontaneous and piezoelectric polarization on the base resistance and frequency response of wurtzite Aluminium Gallium Nitride / Silicon Carbide Heterojunction Bipolar Transistors. Heterojunction Bipolar Transistors ( HBTs ) built using wide band gap semiconductors with AlGaN emitter and SiC base/collector hold the promise of high-power and high-frequency operation due to lower impact ionization coefficients and higher breakdown voltages. Further, Silicon Carbide has an indirect bang gap, and a large lifetime of minority carriers compared to most other compound semiconductors, which tend to have direct band gaps. This reduces the base recombination factor when the base is made from SiC, and helps to achieve higher overall current gain. Spontaneous and piezoelectric polarizations of the order of 1013 electrons per cm2 exist in wurtzite wide band-gap semiconductors. This has a non-trivial effect on band profile, charge transport and overall device characteristics since the polarization-induced charges are of the same order of magnitude as the total dopant charge content of critical device layers, and can significantly affect the amount of mobile charge depletion or accumulation in these layers. We calculate the effect of this polarization for both very thin pseudomorphic emitters and for relaxed emitter structures. We present calculations for the cases of Si-face and C-face SiC, since the signs of polarization-induced charges are different for the two cases. The intrinsic base resistance near emitter flat-band conditions is changed by a factor of 10 depending on the alloy composition of the emitter and the polarity of growth. The maximum frequency of oscillation under emitter flat-band conditions can also be modulated by the polarization-induced charges by up to 60%. Our calculations show that the technologically less prevalent C-face SiC can give a higher advantage for frequency response, especially when the emitter thickness is larger than the critical thickness.

Analytical performance evaluation of AlGaN/GaN metal insulator semiconductor heterostructure field effect transistor and its comparison with conventional HFETs for high power microwave applications

AbstractIn this work, a comprehensive analytical model for AlGaN/GaN MISHFET has been presented to evaluate the drain current characteristics, transconductance, and cut‐off frequency of the insulated device. The model takes into account polynomial dependence of sheet carrier density on position of quasi Fermi level to consider the quantum effects and to validate it from subthreshold to high conduction region. The effect of spontaneous and piezoelectric polarization at the AlGaN/GaN interface and parasitic source/drain resistances have also been incorporated in the analysis. Its advantages over conventional HFET structure are discussed in detail. For a MISHFET with quarter micron gate length, the cut‐off frequency is reported to be 52 GHz. The MISHFET shows remarkable 36% increase in drain saturation current. The model has a broad utility as it is equally applicable to HFETs as well. The present model is based on closed form expression and does not involve any fitting parameter. The results obtained are compared with experimental data and show excellent agreement, thereby proving the validity of the model. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 331–338, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23073

Growth and characterization of free-standing zinc-blende (cubic) GaN layers and substrates

In this paper, we describe bulk, free-standing, zinc-blende (cubic) GaN wafers grown by plasma-assisted molecular beam epitaxy. We have grown GaN layers of up to 60 µm in thickness. We present the data from characterization measurements that confirm the cubic nature of the GaN crystals and show that the fraction of the material that is hexagonal in nature is not more than about 10% in the best thick samples. Cubic (0 0 1) GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0 0 0 1) c-axis wurtzite GaN. Therefore, the free-standing GaN wafers we have grown would make ideal lattice-matched substrates for the growth of cubic GaN-based structures for blue and ultraviolet optoelectronic devices, and high-power and high-frequency electronic applications.

Improved temperature model of AlGaN/GaN HEMT and device characteristics at variant temperature

In this paper, an improved temperature model for AlGaN/GaN high electron mobility transistor (HEMT) is presented. Research is being conducted for a high-performance building block for high frequency applications that combine lower costs with improved performance and manufacturability. The effects of channel conductance in the saturation region and the parasitic resistance due to the undoped GaN buffer layer have been included. The effect of both spontaneous and piezoelectric polarization induced charges at the AlGaN/GaN heterointerface has been incorporated. The proposed model is used to determine the transfer characteristics, output current-voltage characteristics and small-signal microwave parameters of HEMTs. The investigated temperature range is from 100–600 K. The small signal microwave parameters have been evaluated to determine the unity current gain cut-off frequency (f T ). High f T (10–70 GHz) values and high current levels (∼550 mA/mm) are achieved for a 1 μm AlGaN/GaN HEMTs. A custom DC measurement system is used to facilitate the DC characterization of the unpackaged GaN HEMT test device. The calculated critical parameters and the simulation results suggest that the performance of the proposed device degrades at elevated temperatures.

Two dimensional electron gas in InN-based heterostructures: Effects of spontaneous and piezoelectric polarization

This paper predicts and describes the formation of two dimensional electron gas (2DEG) in InN-based heterostructures, as a promising candidate for future high performance high electron mobility transistor (HEMT). The effects of spontaneous and piezoelectric polarization on carrier confinement and localization of 2DEGs have been studied. These include the theoretical analysis and calculation of spontaneous and piezoelectric polarization, and polarization induced sheet charge bound at the interfaces of InN/InGa(Al)N/InN heterostructures as a function of lattice mismatch induced strain. Significant effects of polarity are found on carrier confinement and localization of 2DEGs. A high sheet carrier concentration and strong electron confinement at specific interfaces of the InN-based heterostructures are predicted as a consequence of piezoelectric and spontaneous polarization effects. The calculated sheet carrier concentration reaches as high as 2.12 × 10 14 and 7.08 × 10 13 cm −2 for AlN and GaN barriers, respectively. These sheet carriers generated in InN-based heterostructures are higher than the reported values for the conventional GaN-based AlGaN/GaN HEMTs. The accumulation of electrons can be confirmed for a barrier thickness of above 4 nm.

An analytical two-dimensional model for AlGaN/GaN HEMT with polarization effects for high power applications

An analytical two-dimensional model for AlGaN/GaN modulation-doped field effect transistor is developed. The spontaneous and piezoelectric polarization effects have been included. Two-dimensional analysis has been carried out in the high field region. The output characteristics, device transconductance and cut off frequency for 120 nm gate length device are obtained. Peak transconductance of 320 mS/mm and a cut off frequency of 120 GHz has been obtained. The results show excellent agreement when compared with experimental data thereby proving the validity of the model.

A compact C–V model for 120 nm AlGaN/GaN HEMT with modified field dependent mobility for high frequency applications

We present a theoretical model of AlGaN/GaN high electron mobility transistor (HEMT) that includes the effect of spontaneous and piezoelectric polarization. Present model also incorporates the effect of mole fraction dependent mobility, saturation velocity and the accurate 2-DEG density in HEMT as a function of gate voltage in subthreshold, linear and saturation regimes. This paper reports a detailed 2-D analysis of capacitance–voltage (C–V) characteristics. The contribution of various capacitances including fringing field capacitance on the performance of the device is also shown. The model further predicts the transconductance, drain conductance and frequency of operation and is in close proximity with the experimental data which confirms the validity of proposed model.

Cathodoluminescence of single ZnO nanorod heterostructures

AbstractOptical properties of ZnO‐based single nanorods are probed by cathodoluminescence (CL) measurements at T = 5 K. We observe a variation of the ZnO near band edge CL by three orders of magnitude along the nanorod axis, accompanied by a spectral blueshift of 10–30 meV. This indicates a rather poor structural quality of the nanorod bottom part, close to the substrate. ZnO/ZnMgO quantum wells grown on top of ZnO nanorods are found to exhibit much stronger confinement effects as compared to their two‐dimensional counterparts, suggesting a reduced spontaneous and piezoelectric polarization effects. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Elastic and Piezoelectric Characteristics of Wurtzite GaN/AlN Semiconductor Quantum Dots

The elastic deformation and piezoelectric field in GaN /AlN/ quantum dots have been investigated. The electronic levels of quantum dots have been given in this paper. The 3D strain field and piezoelectric potential are calculated based on and the effective mass theory and finite element method. The effect of spontaneous and piezoelectric polarization is taken into account in the calculation. The ground bound state and the several lowest excited states of quantum dots have been studied. It is found that the size of quantum dots controls the conduction band edge, electronic levels and more other optical properties. The calculation results are very helpful in designing high quality infrared photodetector and laser.

Polarization dependent analysis of AlGaN/GaN HEMT for high power applications

Polarization dependent analysis for AlGaN/GaN HEMT has been done. The capacitance–voltage characteristics of lattice mismatched AlGaN/GaN modulation doped field effect transistor are obtained using charge controlled analysis for its microwave performance. The model includes the spontaneous and piezoelectric polarization effects and device transconductance and cutoff frequency are calculated. The results show excellent agreement when compared with experimental data thereby proving the validity of the model.

Electron Energy Spectra of Single and Multiple AlGaN/GaN Quantum Dots with Spontaneous and Piezoelectric Polarization Effects

AbstractWe present numerical calculations of electron spectra of single and multiple coupled quantum dots based on Aluminium Gallium Nitride / Gallium Nitride heterostructures. The effect of spontaneous and piezoelectric polarization on the confinement potential seen by the electrons is taken into account through bound interface sheet charges. We also calculated the spectra without polarization effects for reference. For some quantum dot dimensions, the energy eigenvalues shift by several hundred meVs due to the polarization charges. We calculate the spectra for the two cases of box-shaped and cylindrical quantum dots. The latter case is an approximation to quantum dots with hexagonal-facet shapes recently reported in the literature. The quantum dots in our calculations are surrounded by vacuum in the lateral direction, but the same qualitative conclusions will hold if the dots are embedded in some material, as long as the barrier heights are large. For GaN vertical confinement of less than 30 angstroms, most of the bound states are associated with the lowest eigenvalue of the vertical confinement potential. This is also true for higher vertical confinement dimensions because the triangular potential seen by the electrons is the same for the lowest energy eigenstates. The electric field in the vertical direction is a strong function of the aluminium concentration in the AlGaN layer. As the AlGaN layer composition is varied from very high Al concentration to medium Al concentration, the spectra shift by several hundred meVs, referred to the onset of the continuous spectra. The transition frequencies between bound states and between bound and the lowest continuum states lie in the low to the high infra-red range, and can be varied over a wide range by both the dimensions and the barrier aluminium concentration. For the case of 4 coupled quantum dots formed by repeated AlGaN/GaN heterojunctions, we find that the polarization-induced electric fields lead to excessive band-bending and as a consequence there are fewer bound states compared to the spectrum calculated without polarization effects.

Variational Calculations of Donor Binding Energy in Rectangular Wurtzite Aluminium Gallium Nitride / Gallium Nitride Quantum Wires

AbstractWe present variational calculations of donor binding energy in rectangular wurtzite aluminium gallium nitride / gallium nitride quantum wires. We explicitly take into account the effect of spontaneous and piezoelectric polarization on the energy levels of donors in quantum wires. Wurtzite structure nitride semiconductors have spontaneous polarization even in the absence of externally applied electric fields. They also have large piezoelectric polarization when grown as pseudomorphic layers. The magnitude of both polarization components is of the order of 1013 electrons per cm2, and has a non-trivial effect on the potential profile seen by electrons. Due to the large built-in electric fields resulting from the polarization discontinuities at heterojunctions, the binding energies of donors is a strong function of the position inside the quantum wire. The potential profile in the 0001 direction can vary by as much as 1.5eV due to polarization effects for vertical dimensions of the quantum wire up to 20 angstroms. The probability density of electrons tends to concentrate near the minimum of the conduction band profile in the 0001 direction. Donors located close to this minimum tend to have a larger concentration of electron density compared to those located closer to the maximum. As a consequence, the binding energy of the former are higher compared to the latter. We use Lorentzian variational wavefunctions to calculate the binding energy as a function of donor position. The confinement potential enhances the binding by a factor of about 3 compared to donors in bulk nitride semiconductors, from about 30 meV to about 90 meV. The variation of binding energy with position is calculated to be more than 50% for typical compositions of the quantum wire regions. Our calculations will be useful for understanding device applications involving n-type doped nitride semiconductor quantum wires.